[Haskell-cafe] Why superclass' instances are bad idea?

John Lato jwlato at gmail.com
Wed Sep 25 07:14:37 CEST 2013

This line

    instance Monad m => Applicative m where

tells the compiler "Every type (of the appropriate kind) is an instance of
Applicative.  And it needs to have a Monad instance as well."

That's what Edward means when he said that it means "every Applicative is a
Monad".  Theoretically the statement makes no sense, but that's what this
instance head means.  Everything is Applicative, and it also needs a Monad
instance to use that Applicative.

Consider what happens for something that isn't a Monad, e.g. ZipList.
Since it's not a Monad, it would need its own instance

    instance Applicative ZipList where

But now you'd need to enable OverlappingInstances, because ZipList matches
both this instance and the general one you've defined above (GHC doesn't
consider constraints when matching instance heads).  OverlappingInstances
is much more problematic than the other extensions because it could (and
almost certainly would in this case) give rise to incoherence (see the
warning under

You might want to read the wiki page on default superclass instances (
http://ghc.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances) for
further discussion of this problem.

John L.

On Tue, Sep 24, 2013 at 12:17 PM, Wvv <vitea3v at rambler.ru> wrote:

> I suggest to add superclass' instances into  libraries.
> http://ghc.haskell.org/trac/ghc/ticket/8348
> In brief, we could write next:
> >{-# LANGUAGE FlexibleInstances #-}
> >{-# LANGUAGE UndecidableInstances #-}
> >
> >instance Monad m => Applicative m where
> >    pure  = return
> >    (<*>) = ap
> >
> >instance Monad m => Functor m where
> >    fmap = liftM
> >
> >instance Monad m => Bind m where
> >    (>>-) = flip (>>=)
> >    B.join = M.join
> this code is valid!
> I've already defined 3 "superclassses" for Monad: Functor, Applicative and
> Bind!
> Similar idea said Edward Kmett in 2010 (founded by monoidal) (
> http://stackoverflow.com/questions/3213490/how-do-i-write-if-typeclass-a-then-a-is-also-an-instance-of-b-by-this-definit/3216937#3216937
> )
> And he said "but effectively what this instance is saying is that every
> Applicative should be derived by first finding an instance for Monad, and
> then dispatching to it. So while it would have the intention of saying that
> every Monad is Applicative (by the way the implication-like => reads) what
> it actually says is that every Applicative is a Monad, because having an
> instance head 't' matches any type. In many ways, the syntax for 'instance'
> and 'class' definitions is backwards."
> Why? I don't understand.
> Not every Applicative is a Monad, but every Monad is Applicative
> --
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